U.S. patent number 10,591,822 [Application Number 16/046,973] was granted by the patent office on 2020-03-17 for imaging device.
This patent grant is currently assigned to LANDA LABS (2012) LTD.. The grantee listed for this patent is Landa Labs (2012) Ltd.. Invention is credited to Ofer Aknin, Tamar Kashti, Benzion Landa, Michael Nagler, Nir Rubin Ben Haim, Itai Tzur, Ronen Yogev.
United States Patent |
10,591,822 |
Rubin Ben Haim , et
al. |
March 17, 2020 |
Imaging device
Abstract
An imaging device for projecting individually controllable laser
beams onto an imaging surface movable in an X-direction. The device
includes a plurality of semiconductor chips each comprising a
plurality of laser beam emitting elements arranged in a main array
of MN. The chips are mounted such that each pair of adjacent chips
in the Y-direction are offset from one another in the X-direction
and, if activated continuously, the emitted laser beams of the two
chips of said pair trace on the imaging surface a set of parallel
lines that are substantially uniformly spaced in the Y-direction.
In addition to the MN elements of the main array, each chip
comprises at least one additional column on one or each side, each
additional column containing at least one selectively operable
element capable of compensating for any misalignment in the
Y-direction in the relative positioning of the adjacent chips on
the support.
Inventors: |
Rubin Ben Haim; Nir (Hod
HaSharon, IL), Nagler; Michael (Tel Aviv,
IL), Landa; Benzion (Nes Ziona, IL),
Kashti; Tamar (Nes Ziona, IL), Aknin; Ofer
(Petach Tikva, IL), Yogev; Ronen (Kibbutz Hulda,
IL), Tzur; Itai (Kibbutz Na'an, IL) |
Applicant: |
Name |
City |
State |
Country |
Type |
Landa Labs (2012) Ltd. |
Rehovot |
N/A |
IL |
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Assignee: |
LANDA LABS (2012) LTD.
(Rehovot, IL)
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Family
ID: |
56296864 |
Appl.
No.: |
16/046,973 |
Filed: |
July 26, 2018 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20180329306 A1 |
Nov 15, 2018 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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15363129 |
Nov 29, 2016 |
10061200 |
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PCT/IB2016/053137 |
May 27, 2016 |
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PCT/IB2016/053138 |
May 27, 2016 |
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Foreign Application Priority Data
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May 27, 2015 [GB] |
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1509073.1 |
May 27, 2015 [GB] |
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1509077.2 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G03G
15/04072 (20130101); G03F 7/70025 (20130101); B41J
2/45 (20130101); B41J 2/447 (20130101); B41J
2/455 (20130101); G03G 15/043 (20130101); B41J
2/451 (20130101); G03G 15/342 (20130101) |
Current International
Class: |
G03F
7/20 (20060101); B41J 2/447 (20060101); B41J
2/45 (20060101); B41J 2/455 (20060101); G03G
15/04 (20060101); G03G 15/043 (20060101); G03G
15/34 (20060101) |
References Cited
[Referenced By]
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Other References
Machine translation of JPS5726874. cited by applicant .
Machine Translation of JP2009056795A. cited by applicant .
Machine Translation of JPS5557801A. cited by applicant .
Machine Translation of JP2009149051. cited by applicant .
Machine Translation of JP2008074052A. cited by applicant .
Machine Translation of JP2006186192. cited by applicant .
Machine Translation of JPS6299166A. cited by applicant .
Machine Translation of JP2009158477A. cited by applicant.
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Primary Examiner: Valencia; Alejandro
Attorney, Agent or Firm: Wertsberger; Shalom Saltamar
Innovations
Claims
The invention claimed is:
1. A method of projecting individually controllable laser beams
onto an imaging surface that is movable relative to an imaging
device, the method comprising: providing an imaging device for
projecting individually controllable laser beams onto an imaging
surface, the imaging device and imaging surface being movable
relative to each other in a reference X-direction, the imaging
device comprising: a plurality of semiconductor chips each of which
comprises a plurality of individually controllable laser beam
emitting elements arranged in a two dimensional main array of M
rows and N columns, the emitting elements in each row having a
uniform spacing A.sub.r and the emitting elements in each column
having a uniform spacing a.sub.c; the chips are mounted on a
support such that the main arrays of each pair of chips that are
adjacent one another in a reference Y-direction, transverse to the
X-direction, are offset from one another in the X-direction;
wherein were all the emitting elements to be activated continuously
and were the chips and the imaging surface to be relatively moved
in the X-direction, the emitted laser beams of the two chips of the
pair would trace on the imaging surface 2MN parallel lines that
extend in the X-direction and are uniformly spaced from one another
in the Y-direction, by a nominal distance A.sub.r/M, whereby the
laser beams of each chip trace a set of MN lines without
overlapping the set of lines of the other chip; each chip further
comprises at least one additional column in addition to the N
columns of elements of the main array, the additional column being
disposed on one side of the main array along the Y-direction, and
containing at least one selectively operable laser emitting element
capable of tracing at least one additional line that lies between
the respective sets of MN lines of each chip and that is spaced
from two adjacent lines, each from a respective one of the sets, by
a distance smaller than the uniform element spacing in each row
A.sub.r; and, individually projecting the laser beams onto the
imaging surface.
2. A method as claimed in claim 1, further comprising the step of
controllably projecting a laser beam emanating from the at least
one emitting element of the at least one additional column of a
first of two adjacent chips, the laser beam impinging on the
imaging surface between laser beams impinging on the imaging
surface from at least two laser beams emanating from emitting
elements at the edge of the respective main arrays of the first and
a second of two adjacent chips.
3. A method as claimed in claim 2, wherein the additional column of
the first chip of the pair of chips and the additional column of
the second chip are disposed between the respective main arrays of
the first and second chip, the method further comprising the step
of controllably projecting a first laser beam emanating from one
emitting element of the additional column of the first chip and a
second laser beam emanating from the additional column of the
second chip, such that the first and second laser beams impinge on
the imaging surface at sufficient proximity to cause the energy of
the respective laser beams to act additively thereupon.
4. A method as claimed in claim 3, further comprising the step of
controlling the energy of each of the first and second laser beams
such that each of the beams act on the imaging surface at an energy
level below a predetermined threshold, and the additively combined
energy level of the two beams surpasses the predetermined
threshold, the additively combined energy level impinges on the
imaging surface at a location between the respective first and
second beam centers.
5. A method as claimed in claim 1, further comprising the step of
controllably projecting a first and second laser beams emanating
respectively from two emitting elements adjacent in the Y
direction, at controlled intensity such that the first and second
laser beams impinge on the imaging surface at sufficient proximity
and intensity to cause the energy of the respective beams to act
additively thereupon.
6. A method as claimed in claim 5, further comprising the step of
controlling the energy of first and second laser beams such that
each of the beams act on the imaging surface at an energy level
below a predetermined threshold, and the additively combined energy
level of the first and second laser beams surpasses the
predetermined threshold, the additively combined energy level
impinges on the imaging surface at a location between the
respective first and second beam centers.
7. A method as claimed in claim 1, wherein the at least one
additional column comprises a plurality of emitting elements.
8. A method as claimed in claim 1, wherein the imaging device
further comprises a plurality of lens systems, each serving to
focus the laser beams of all the emitting elements of a respective
chip onto the imaging surface.
9. A method as claimed in claim 8, wherein at least one of the
plurality of lens systems comprises at least one gradient index
(GRIN) rod.
10. A method as claimed in claim 9, wherein the at least one GRIN
rod is of circular cross-section having a diameter equal to
2NA.sub.r.
11. A method as claimed in claim 1, wherein each chip comprises at
least a second additional column, such that at least one additional
column is disposed on each side of the respective main array, the
second additional column comprising at least one individually
controllable laser beam emitting element.
12. A method as claimed in claim 1, wherein each chip has an equal
number of rows and columns of emitting elements in the main
array.
13. A method as claimed in claim 1, wherein the surface of the
support is formed of, or coated with, an electrical insulator, and
further comprising a plurality of thin film conductors formed on
the electrically insulating surface for supplying electrical
signals and power to the chips.
14. A method as claimed in claim 1, wherein the support is liquid
cooled.
15. A method as claimed in claim 1, wherein the emitting elements
of at least one chip are of vertical cavity surface emitting lasers
(VCSEL) type.
Description
RELATED APPLICATIONS
This Patent Application incorporates by reference in their entirety
International Patent Applications Nos. PCT/IB2016/053138 and
PCT/IB2016/053137, filed on May 27, 2016, and GB Patent
Applications Nos. 1509073.1 and 1509077.2, filed on May 27,
2015.
FIELD
The present disclosure relates to an imaging device for projecting
a plurality of individually controllable laser beams onto a surface
that is movable relative to the imaging device.
BACKGROUND
U.S. Pat. No. 7,002,613 describes a digital printing system to
which the imaging device of the present disclosure is applicable,
by way of example. In particular, in FIG. 8 of the latter patent
specification, there is shown an imaging device designated 84 that
is believed to represent the closest prior art to the present
disclosure. The imaging device serves to project a plurality of
individually controllable laser beams onto a surface, herein termed
an imaging surface, to generate an energy image onto that surface.
The laser image can be used for a variety of purposes, just a few
examples being to produce a two dimensional printed image on a
substrate, as taught for instance in U.S. Pat. No. 7,002,613, in 3D
printing and in etching of an image onto any surface.
For high throughput applications, such as commercial printing or 3D
lithography, the number of pixels to be imaged every second is very
high, demanding parallelism in the imaging device. The laser
imaging device of the present disclosure is intended for
applications that require energy beams of high power where the
total power required can be of tens or hundreds of milliwatt (mW).
For instance, in the field of printing, depending on the desired
printing speed, the energy beams can provide powers of up to 10 mW,
100 mW and even 250 mW or higher. One cannot therefore merely scan
the imaging surface with a single laser beam, so as to expose the
pixels sequentially. Instead, the imaging device is required to
have a plurality of laser emitting elements for various pixels
(picture elements) each laser capable of tracing a line of pixels
in the image area of an imaging surface in relative motion.
To achieve acceptable print quality, it is important to have as
high a pixel density as possible. A high resolution image, for
example one having 1200 dpi (dots per inch), requires a density of
laser emitting elements that is not achievable if the laser
emitting elements all lie in a straight line, due to the amount of
overlap necessary between the laser sources to achieve a uniform
printing quality. Aside from the fact that it is not physically
possible to achieve such a high packing density, adjacent elements
would interfere thermally with one another.
Semiconductor chips are known that emit beams of laser light in an
array of M rows and N columns. In U.S. Pat. No. 7,002,613 the rows
and columns are exactly perpendicular to each other but the chips
are mounted askew, in the manner shown in FIG. 1 of the latter
patent, so that each row can fill in the missing pixels of the
preceding row(s). In this way, such an array can achieve a high
resolution image but only over the width of the chip and such chips
cannot simply be mounted side by side if one is to achieve a
printed image without stripes along its length, because the chips
cannot have laser emitting elements positioned sufficiently close
to their lateral edges.
U.S. Pat. No. 7,002,613 avoids this problem by arranging such chips
in two rows, in the manner shown in FIG. 8 of the latter patent.
The chips in each row are staggered relative to the chips in the
other row of the pair so that each chip in one row scans the gap
left unscanned by the two adjacent chips in the other row.
Even though it is expected that the rows of chips will be mounted
on a support under clean laboratory conditions using a microscope
to achieve their correct alignment, it is guaranteeing that the
relative alignment of the chips in the two rows will be accurate
within the resolution of the printed image is difficult and
expensive. Any misalignment will result in the image having stripes
or other undesired defects.
US 2010/080594 and US 2008/181667 describe systems in which the
light from arrays of LED's (rather than laser sources) is projected
onto an image surface and teach how steps may be taken to
compensate for any misalignment between the arrays. In each case,
the images produced by adjacent arrays are overlapped and selected
LED's from one or other of the two arrays are activated to maintain
image continuity at the boundary between the two arrays. In the
case of US 2010/080594 this overlap is shown clearly in FIG. 14 and
in US 2009/181667 it is evident, for example, from FIGS. 9A and
9B.
SUMMARY
In the present disclosure, there is disclosed an imaging device for
projecting individually controllable laser beams onto an imaging
surface that is movable relative thereto in a reference
X-direction, the device including a plurality of semiconductor
chips each of which comprises a plurality of individually
controllable laser beam emitting elements arranged in a two
dimensional main array of M rows and N columns (MN), the elements
in each row having a uniform spacing A.sub.r and the elements in
each column having a uniform spacing a.sub.c, wherein the chips are
mounted on a support in such a manner that when nominally placed,
each pair of chips that are adjacent one another in a reference
Y-direction, transverse to the X-direction, are offset from one
another in the X-direction, and such that the center of laser beam
emitting elements of the main MN emitting elements arrays of both
chips in the pair are nominally uniformly spaced in the Y-direction
by a nominal distance A.sub.r/M, without overlap in the Y-direction
between the beam emitting elements of the adjacent chips. Stated
differently, were all the laser emitting elements of the pair of
nominally placed adjacent chips to be activated continuously, and
were the chips and imaging surface to be in relative motion in the
X-direction, the emitted laser beams of the respective main arrays
of the two chips of the pair would trace on the imaging surface a
set of parallel lines that extend in the X-direction and that are
nominally uniformly spaced in the Y-direction. The lines traceable
by emitting elements of the first chip would not interlace with the
lines traceable by emitting elements of the second chip.
As a major object of the invention involves compensating for minor
misalignment of the chips, it is important to realize that the
disclosure of relative placement relates to the desired positioning
within certain tolerances that enables satisfactory results from
the imaging device. Therefore, the term "nominally", should be
construed to mean that the stated spatial relationship exist when
the chips or other relevant elements are disposed at their intended
placing. However, different aspects of the invention allow
compensating for chip placements that diverge from that nominal
position. Similarly, when used to indicate spatial relationship the
term "beam" should be considered as relating primarily to the
center of the beam, unless otherwise indicated or clear from the
context. Thus by way of example the uniform spacing A.sub.r and
a.sub.c relate to the distance between the centers of the laser
beam emitting elements.
In order to compensate for minor misalignment, in addition to the M
rows and N columns of elements of the main array, each chip
comprises at least one additional column on at least one side of
the main array, each such additional column containing at least one
selectively operable laser emitting element disposed for tracing at
least one additional line that lies between the two sets of MN
lines. This element, also termed the additional element or the
alignment element, is thus capable of compensating for some
misalignment in the Y-direction in the relative positioning of the
adjacent chips on the support.
Assuming that the M rows and N columns of laser emitting elements
of the main array do not include any elements that are normally
redundant, the spacing between adjacent lines in the set will be
equal to A.sub.r/M, namely the spacing of the adjacent elements in
each row divided by the number of rows. Furthermore, because in the
present disclosure there is no overlap between the two sets of MN
lines traced by any two adjacent chips, the total number of lines
traced by the two chips will be equal to 2MN, namely twice the
product of the number of rows and the number of columns in each
chip, if the chips have equal numbers of rows and columns.
In an aspect of the invention, in addition to these evenly spaced
lines produced by the main MN arrays, additional laser emitting
elements are provided on at least one end, or on both ends of each
array, intended only to compensate for chip misalignment. If
adjacent chips are correctly aligned, the elements of the
additional columns will be redundant and will not be energized.
However, if a gap should remain between the lines traced by
adjacent chips, the additional elements can introduce additional
lines to fill that gap at a position approximating the uniform
spacing of the lines traced by the main MN arrays. It should be
noted that, in contrast to the proposals in US 2010/080594 and US
2008/181667, the lines traced by the additional elements do not
fall between (i.e. are not interlaced with) the lines of either set
of MN lines traced by the main arrays and only fall within any gap
between the two sets of MN lines.
Were the imaging surface moved relative to laser beams emitted by
adjacent laser elements, the laser radiation centered on each line
traced in the X-direction, would have a non-uniform energy profile
which typically, but not necessarily, approaches a Gaussian
intensity distribution. The spot size traced can be made large
enough so that the energy traced by one laser element overlaps the
area traced by an adjacent element and the intensity combination of
the two beams, as well as the control over the amplitude of one or
both beams, offers a combined intensity profile whose maximum may
be moved between the two adjacent traced lines by controlling the
relative intensity, and/or timing, thus placing an intermediate
line traced at a selectable position between the two original line
centers.
In the event of an overlap between the two sets of MN lines traced
by the elements of adjacent chips, some of the elements of the main
arrays can be switched off and if necessary replaced by an element
of the additional columns to maintain the appearance of a raster
with uniformly spaced lines.
On the other hand, if a gap exists between the two MN lines traced
by the elements of adjacent chips, the additional columns can be
activated to maintain the appearance of a raster with uniformly
spaced lines.
One of the additional elements may be activated on its own if its
position coincides with a line that would render the raster
uniform. Alternatively, if the elements have a symmetrical energy
profile, resembling for example a Gaussian or a sinusoidal
distribution, it is possible, by activating two elements to
irradiate adjacent spots on the imaging surface and separately
adjusting the power of each element, to produce a single raster
line at an adjustable distance from the raster lines of the main
elements of the two chips. It should be noted that this effect is
thermally dynamic and additive provided that the adjacent spots are
irradiated within a finite time of each other. In other words, the
imaging surface should not have time to dissipate the energy of the
first laser pulse in the interval between the two laser pulses.
Furthermore, the two elements may be either on a single additional
column of one chip or on additional columns each residing on a
separate chip, assuming that the additional columns are disposed
between the respective main arrays of the two chips. Chips having
additional columns on both sides of the main arrays would provide
such arrangement of the additional columns of the two chips being
disposed between the respective chips main arrays.
Conveniently, the lines traced by elements in the additional column
are evenly spaced from one another, the spacing between the lines
traced by the element of the additional column being substantially
equal to the quotient of the spacing of the lines traced by the
elements of main array and the number of elements in the additional
column.
While it would be possible to use chips in which the rows and
columns of the main arrays of laser emitting elements are
perpendicular to one another, as taught in U.S. Pat. No. 7,002,613,
doing so requires the chips to be placed at an angle relative to
the Y-direction. In some embodiments of the present disclosure the
elements in each row of each chip lie on a line parallel to the
Y-direction and the elements in each column of each chip lie on a
straight line inclined at an angle to the X-direction. In other
words, instead of the outline of the array being square, the array
is shaped as a parallelogram. This arrangement, which may be
considered slightly wasteful as far as chip area is concerned, can
be advantageous in terms of assembling procedure.
It is convenient for the chips to be arranged in at least one pair
of rows on the support, with corresponding laser emitting elements
of all the chips in each of the two rows lying in line with one
another in the Y-direction. By "corresponding elements" it is meant
that the individual laser emitting elements of the MN main array
should occupy the same row and column positions within their
respective chips. It is advantageous for corresponding elements in
any group of three chips in the pair of rows that are adjacent one
another in the X and Y-directions to lie at the apices of congruent
equilateral triangles. This arrangement simplifies the construction
of the lens system to focus the laser beams onto the imaging
surface.
It has been found particularly advantageous for all the laser beams
emitted by one chip to be focused on the imaging surface by a
common single lens, or a common set of lenses arranged in series,
having a magnification M.sub.o whose absolute value is greater than
or equal to one (1), however magnification lower than one (1) is
also explicitly considered. It was found to be even more
advantageous if the magnification M.sub.o was substantially equal
to +1, as that would ensure that the laser elements can be spaced
adequately on the chip even for high resolution systems. Stated
differently, the image of the array of laser elements on the
imaging surface (i.e. an array of dots) would have the same size as
the array on the chip, though it may be inverted with a
magnification of -1. Notably, even if a slight misalignment of the
lenses exists, such as GRIN rod (Gradient-Index) lenses, in the XY
plane perpendicular to the optical axis of the lens, the position
of the illuminated laser spot on the imaging surface will remain
unchanged, as it only depends on the position of the laser emitting
element on the laser array chip. The former elements can be
positioned with very high accuracy on every laser array chip using
standard semiconductor manufacturing techniques.
While the lens system may comprise a single GRIN rod associated
with each chip, it may alternatively comprise a plurality of GRIN
rods arranged in series with one another and forming a folded light
path where the fold is in the space where a beam emitted by the
laser elements is substantially individually collimated. In folded
light path embodiments, a reflecting member such as a prism or
mirror which is optionally common to all the chips may serve to
direct the laser beams from one GRIN rod element to the next in
each series. In such a folded light path configuration, it is
desirable for the reflecting member to be on a facet of a folding
prism made of a material, typically a glass, having a higher
refractive index than the highest refractive index in the GRIN
rods. The higher index of refraction of the prism will limit the
angular divergence of the collimated beams and allow larger
separation between the sequential GRIN rod segments. A suitable
light path folding prism can be for example a right angle prism,
the folding face of the prism being a reflecting surface. Other
types of reflecting members and folding angles can be used
depending on the geometry of the system and the direction to be
given to beams in the series.
It is convenient for the main array of each chip to have an equal
number of rows and columns of laser beam emitting elements (i.e.,
M=N), as this minimizes the size of the lens system.
Within each chip, the separation between the laser elements is
desirably sufficiently great to minimize thermal interference
between adjacent laser emitting elements.
The support for the chip arrays may be fluid cooled to help
dissipate the heat that may be generated by the chips.
In certain embodiments, the support may be a rigid metallic or
ceramic structure and it may be formed of, or coated with, an
electrically insulating surface bearing film conductors to supply
electrical signals and power to the chips.
The chips in some embodiments are vertical cavity surface emitting
laser (VCSEL) chip arrays. Equivalently other types of laser
sources may be utilized and the term VCSEL should be construed as
encompassing such laser sources.
In some embodiments, the intensity of the laser beam emitted by
each element may be adjustable either continuously (in an analogue
manner) or in discrete steps (digitally). In one embodiment, the
chips may include D/A converters so as to receive digital control
signals. In this way, the laser beam intensity may be controllably
adjusted in a plurality of discrete steps, such as 2, 4, 8, 16, 32,
. . . 4096 and the like.
In a further aspect of the present disclosure, there is provided a
method of projecting individually controllable laser beams onto an
imaging surface that is movable relative to an imaging device
utilizing the imaging device of any embodiment of the present
disclosure, so as to form an image comprising pixels or lines
thereof when the projected laser beam is intermittent or
continuous, respectively.
In some embodiments, at least one pair of laser elements, selected
either both from the same array or one from each of two adjacent
arrays, are controlled in such a manner that their energies are
combined on the imaging surface to increase the temperature of the
imaging surface above a predetermined threshold at a point
intermediate the centers of the images of the two laser elements on
the imaging surface, without raising the temperature of the imaging
surface at at least one of the centers of the images of the two
laser elements above the latter threshold.
Clearly in operation the laser emitting elements are switched on
and off as needed to provide the required image on the imaging
surface, as continuous operation of all laser beams would result in
a substantially uniformly irradiated surface.
BRIEF DESCRIPTION OF THE DRAWINGS
Some embodiments of the imaging device are described herein with
reference to the accompanying drawings. The description, together
with the figures, makes apparent to a person having ordinary skill
in the art how the teachings of the disclosure may be practiced, by
way of non-limiting examples. The figures are for the purpose of
illustrative discussion and no attempt is made to show structural
details of an embodiment in more detail than is necessary for a
fundamental and enabling understanding of the disclosure. For the
sake of clarity and simplicity, some objects depicted in the
figures are not to scale.
In the Figures:
FIG. 1 is a schematic diagram of a digital printing system
utilizing an imaging device according to an embodiment of the
present disclosure;
FIG. 2 shows part of an imaging device comprising a set of VCSEL
chips mounted on a support;
FIG. 3 is a schematic representation of the laser emitting elements
of two VCSEL chips and the lines that they can trace on a
relatively moving imaging surface;
FIG. 4 is a schematic representation that demonstrates in one pair
of rows the alignment between the VCSEL chips and the GRIN rods
used as lenses to focus the emitted laser beams onto the imaging
surface;
FIG. 5A shows prior art proposals for correction of chip
misalignment;
FIG. 5B shows the manner in which an embodiment of the invention
compensates for chip misalignment;
FIG. 6 shows the energy profiles produced by the laser elements at
the ends of two adjacent arrays, to illustrate how a single line
can be traced using two laterally positioned laser elements, there
being shown for each array three elements of the main array and one
of the additional elements;
FIG. 7A is a similar energy diagram to FIG. 6 to show how the
energies of two adjacent laser elements of the main array can be
combined on the imaging surface to produce an additional dot that
does not fall on the center line of either of the laser
elements;
FIG. 7B shows the dot pattern on the imaging surface produced by
activating four laser elements of the main array in the manner
shown in FIG. 7A;
FIG. 8A shows how the dot pattern of FIG. 7B assists in
anti-aliasing;
FIG. 8B shows for comparison with FIG. 8A the jagged edge that
normally occurs when printing an oblique line; and
FIG. 9 shows an alternative lens system to that shown in FIG. 1
that has a folded light path to permit more compact packaging in a
printing system.
DETAILED DESCRIPTION
The imaging device will be described herein mainly by reference to
its application in digital printing systems however its use is not
limited to this application, and different aspects of the invention
may be implemented to controllably project image forming light
beams onto any surface with relative motion between the surface and
the chips.
Overall Description of an Exemplary Printing System
FIG. 1 shows a drum 10 having an outer surface 12 that serves as an
imaging surface. As the drum rotates clockwise, as represented by
an arrow, it passes beneath a coating station 14 where it acquires
a monolayer coating of fine particles. After exiting the coating
station 14, the imaging surface 12 passes beneath an imaging device
15 of the present disclosure where selected regions of the imaging
surface 12 are exposed to laser radiation which renders the
particle coating on the selected regions of the surface 12 tacky.
Next, the imaging surface passes through an impression station 19
where a substrate 20 is compressed between the drum 10 and an
impression cylinder 22. The pressure applied at the impression
station causes the selected regions of the coating on the imaging
surface 12 that have been rendered tacky by exposure to laser
radiation by the imaging device 15 in the correspondingly termed
imaging station to transfer from the imaging surface 12 to the
substrate 20.
The term "tacky" as used herein is intended to mean that the
irradiated particle coating is not necessarily tacky to the touch
but only that it is softened sufficiently to be able to adhere to
the surface of a substrate when pressed against it in the
impression station 19.
The regions on the imaging surface 12 corresponding to the selected
tacky areas transferred to the substrate 20 consequently become
exposed, being depleted by the transfer of particles. The imaging
surface 12 can then complete its cycle by returning to the coating
station 14 where a fresh monolayer particle coating is applied only
to the exposed regions from which the previously applied particles
were transferred to the substrate 20 in the impression station
19.
Advantageously, a monolayer of particles facilitates the targeted
delivery of radiation as emitted by the laser elements of an
imaging device according to present teachings. This may ease the
control of the imaging device and process, as the selectively
irradiated particles reside on a single defined layer. When
considered for use in a printing system, an imaging device
targeting a monolayer can preferably focus the laser radiation to
form upon transfer to a substrate a dot of approximately even
thickness and/or relatively defined contour.
Reverting to the coating station 14, it may comprise a plurality of
spray heads 1401 that are aligned with each other along the axis of
the drum 10 and only one is therefore seen in the section of FIG.
1. The sprays 1402 of the spray heads are confined within a bell
housing 1403, of which the lower rim 1404 is shaped to conform
closely to the imaging surface leaving only a narrow gap between
the bell housing 1403 and the drum 10. The spray heads 1401 are
connected to a common supply rail 1405 which supplies to the spray
heads 1401 a pressurized fluid carrier (gaseous or liquid) having
suspended within it the fine particles to be used in coating the
imaging surface 12.
The imaging device 15 in FIG. 1 is composed of a support 16
carrying an array of chips each having an arrangement of
individually controlled laser sources capable of emitting laser
beams. In some embodiments, the laser beam emitting elements can
coherently emit light in a range of wavelengths from about 400 nm
to about 12 .mu.m, or up to about 10 .mu.m, or up to about 8 .mu.m,
or up to about 3 .mu.m, or up to about 1.4 .mu.m. Such ranges
includes regions generally known as Near Infra Red (NIR,
.about.0.75-1.4 .mu.m), Short-Wavelength Infra Red (SWIR,
.about.1.4-3 .mu.m), Mid-Wavelength Infra Red (MWIR), also called
Intermediate Infra Red (IIR, 3-8 .mu.m), and Long-Wavelength Infra
Red (LWIR, 8-15 .mu.m), also known as Thermal Infra Red (TIR). In a
particular embodiment, the laser beam emitting elements are NIR
lasers. The laser sources may by way of example, be of VCSEL
(Vertical Cavity Surface Emitting Laser) type, however other types
may be utilized. By way of example, semiconductor lasers
commercially available as laser diodes are capable of emitting at
wavelengths from 375 nm to 3,500 nm, covering most of NIR and SWIR
regions of the spectrum. Gas lasers can emit over various area of
the spectrum, depending on the elected gas and some optical design.
Commercial carbon dioxide (CO.sub.2) lasers, for instance, can emit
hundreds of watts in the thermal infrared region at 10.6 .mu.m.
While for brevity the term VCSEL is predominantly used herein, it
should be construed as encompassing any such laser sources which
may be better suited for certain embodiments.
Each chip has individually controllable laser beam emitting
elements arranged in a two dimensional main array of M rows and N
columns (MN), the elements in each row having a uniform spacing
A.sub.r and the elements in each column having a uniform spacing
a.sub.c. As disclosed below, at least one additional column is also
provided.
Preferably, the chips can be individually or collectively
associated with an array of corresponding lenses 18 that focus the
laser beams on the imaging surface 12 is also used. FIGS. 2 to 4
provide more details of the chips 30 according to some embodiments
of the invention and on the manner in which they can be mounted on
the support and aligned with the lenses 18.
FIG. 2 shows a support 16 on which are mounted a plurality of VCSEL
chips 30 arranged in two rows in accurately predetermined positions
relative to one another, as will be described in more detail by
reference to FIGS. 3 and 4.
The support 16 is a rigid and in some embodiments at least
partially hollow elongate body fitted with connectors 34 to allow a
cooling fluid to flow through its internal cavity. In some
embodiments, the body of the support may be made of an electrically
insulating material, such as a suitable ceramic, or it may be made
of a metal and at least its surface 36 on which the chips 30 are
mounted may be coated with an electrical insulator. This enables a
circuit board made of thin film conductors (partial and symbolic
depiction of the conductors is schematically shown to the
lower-right chip at FIG. 2) to be formed on the surface 36. The
chips 30 are soldered to contact pads on this circuit board and a
connector 32 projecting from the lower edge of the support 16
allows control and power signals to be applied to the chips 30. The
laser emitting elements 40 of each chip 30 are individually
addressable and are spaced apart sufficiently widely to minimize
thermal interference with one another.
In some embodiments, the individually controllable laser elements
of a chip can emit laser beams having variable energy that is
preferably digitally controllable in discrete steps, allowing the
laser intensity to be set at discrete levels such as 2, 4, 8, 16 .
. . and the like, and in some embodiments individual laser beam
sources may be controllably set to emit up to 4096 levels or more.
The lowermost level of energy is defined as 0, where the individual
laser element is not activated, the uppermost level of energy can
be defined as 1. The distinct intermediate levels therebetween may
be considered analogous in the field of printing to "grey levels",
each level providing for a gradually distinct intensity (e.g.,
shade when considering a colored output). Taking for instance, a
laser beam emitting element having 16 levels of activation, level 0
would result in lack of impression (e.g., leaving a substrate bare
or white if originally so) and level 1 would result in transfer of
a tacky film formed by a particle irradiated at maximum energy
(e.g., forming a full black dot in the event the particles are so
colored). In previous illustrative example, levels 1/16, 2/16, 3/16
and so on would correspond to increasingly stronger shades of grey,
comprised between white (0) and black (1). Typically, the energy
levels are evenly spaced.
In an alternative embodiment, the individually controllable laser
elements of a chip can emit laser beams having variable energy that
can be modulated in a continuous analogue manner.
Once a region of the imaging surface has reached a temperature at
which the particles become tacky, any further increase in
temperature will not have any effect on the transfer to the
substrate. However, it should also be noted that as the intensity
of the laser is increased the size of the dot that is rendered
tacky also increases.
The energy profile of each dot resembles the plots shown in FIG. 6,
that is to say that it is symmetrical with tapering sides. The
exact profile is not important as the distribution may be Gaussian,
sinusoidal or even an inverted V. In any such profile, as the peak
intensity increases, the base widens and the area of intersection
of the profile with a threshold at which the particle coating is
rendered tacky also increases in diameter. A consequence of this
energy distribution is that points of the imaging surface that are
not in alignment with the centerline of any one laser emitting
element will receive energy from adjacent elements. It is possible
for two nearby elements to be energized to below the level needed
to render coating particles on the centerline of the elements
tacky, yet for the cumulative energy in the region of overlap
between the two centerlines to rise above the level necessary to
render the coating particles tacky. In this way, it is possible to
create potential raster lines between the centerlines of the laser
lines in addition to, or as an alternative to, the raster lines
coinciding with the centerlines of the laser elements. This ability
to combine the energies from adjacent elements is used to achieve
different effects, as will be described below. These effects are
dependent upon the ability of the imaging surface to combine
energies received from different laser elements, even if there is a
slight difference between the times of irradiation.
FIG. 3 shows schematically, and to a much enlarged scale, the
relative positioning of two laser emitting element arrays 130a and
130b of chips 30 that are adjacent one another in the Y-direction
but are located in different rows. Each of the chips has a main
array of M by N laser emitting elements 40, as previously
described, which are represented by circular dots. In the example
illustrated, M and N are equal, there being nine rows and nine
columns. The spacing between the elements in a row, designated
A.sub.r, and the spacing between the elements in a column,
designate a.sub.c, are shown as being different from one another
but they may be the same. The array is shown as being slightly
skewed so that the columns and rows are not perpendicular to one
another. Instead, the rows lie parallel to the Y-direction while
the columns are at a slight angle to the X-direction. This enables
lines, such as the lines 44, traced by the elements 40 on the
imaging surface, if energized continuously, to be sufficiently
close together to allow high resolution images to be printed. FIG.
3 shows that the element at the end of each row traces a line that
is a distance A.sub.r/M away from the line traced by the
corresponding element of each adjacent row, the separation between
these lines being the image resolution I.sub.r. Thus, assuming a
magnification of |1|, A.sub.r and M are selected in dependence upon
the desired image resolution, based on the equation
A.sub.r=MI.sub.r.
It should be mentioned that it is possible for the elements to lie
in a square array where the columns are perpendicular to the rows.
In this case, the chips would need to be mounted askew on their
support and compensation would need to be applied to the timing of
the control signals used to energize the individual elements.
As is clear from FIG. 3, and also FIG. 5B which shows the traced
lines to a larger scale, the positioning of the array 130b is such
that the line traced by its bottom left element 40 should ideally
also be spaced from the line traced by the top right element of the
array 130a by a distance equal to A.sub.r/M. Therefore when all the
elements 40 of both arrays 130a and 130b are energized, they will
trace 2MN lines that will all be evenly spaced apart by a distance
A.sub.r/M between adjacent lines, without any gaps.
If one wishes to provide compensation for defective elements, the
array could include additional rows of laser emitting elements 40,
but it is alternatively possible to compensate for a defective
element by increasing the intensity of the laser beams generated by
the laser emitting elements that trace the two adjacent parallel
lines.
In addition to the M by N array of elements 40, each chip has at
least one additional column that is arranged along the Y-direction
on the side of the main array, the additional column containing at
least one laser beam emitting element 42. These further elements 42
are represented in FIG. 3 by stars, to distinguish them from the
main array elements 40. As seen in FIG. 4, in some embodiments at
least two such additional columns each of one element 42 are
provided, at least one column disposed in Y direction on each side
of the main N by M array. The additional laser elements of the
additional columns on one or both sides of each main array can be
respectively positioned at a distance of 1/2 or 1/3 the spacing
between traced lines that can be imaged by the lenses onto the
imaging surface. Furthermore additional elements could be placed in
the gap between two arrays that nominally spans a distance of
A.sub.r/M so that higher sensitivity is achieved in correcting the
spacing errors between adjacent arrays.
Any additional element 42 of an additional column can be positioned
in the column at any desired distance from the edge element of the
main array, the distance in the Y-direction depending on the total
numbers of additional elements/additional columns each two sets of
main arrays of a pair of chips to be aligned would bound. Assuming
n additional elements 42 between a first and second main array, n
being a positive integer number, each additional element can be
spaced from the edge element of the main arrays or from one another
in the Y-direction by a distance equal to A.sub.r/(n+1), namely the
spacing of the adjacent elements in each row divided by one more
than the number of additional elements in the gap. Considering now
the X-direction, the additional elements can either be aligned with
a row of elements of their respective main arrays or positioned at
any desired intermediate position above or below such rows.
Preferably the positioning of an additional element 42 with respect
to adjacent elements of the main array shall minimize thermal
interference. Notably, the additional element or elements may be
disposed at any position along the X-direction of the chip.
In practice n elements 42 positioned in any of the additional
columns on one or both sides of the main array, can correct for
alignment errors of up to about a 1/(n+1) of the nominal spacing
between the edge elements of two adjacent chips. If, by way of
example, the edge elements of the two chips are at a distance of 20
.mu.m (micrometers) in the Y-direction, and there is a single
additional laser emitting element on adjacent sides of each array,
such elements may correct a spacing error of up to about one third
of the nominal spacing, in the exemplified case approximately 7
.mu.m. Any positional deviation from the desired position on the
chip (e.g., with respect to its edges) or nominal distance between
elements not exceeding 10%, is considered within tolerances,
however in most cases due to the high precision of the
semiconductor manufacturing methods, such errors are unlikely.
As can be seen from FIG. 3 and FIG. 5B, when activated, these
elements 42 trace two additional lines 46 between the two sets of
evenly spaces parallel lines 44a and 44b traced by the elements 40
of the two arrays 130a and 130b, respectively.
One of the additional lines 46 is spaced by a distance A.sub.r/3M
from the last adjacent line 44a traced, for example, by the array
130a in FIG. 3 and the other is spaced by a distance A.sub.r/3M
from the first adjacent line 44b traced, for example, by the array
130b. In the event of a misalignment between the two arrays 130a
and 130b these elements 42 can be energized in addition to, or
instead of some of, the elements 40 of the main arrays to
compensate for any misalignment between the arrays 130a and 130b
that tends to create a stripe in the printed image, be it a gap or
a dark line resulting from an unintentional overlap. FIG. 5A, which
is similar to FIG. 5B, shows the alternative approach proposed in
the prior art to compensate for chip misalignment. In the prior
art, each chip has an additional row of elements that produces
traced lines that are interlaced with the traced lines of the
adjacent chip, resulting in a very high degree of redundancy.
While the two additional elements 42 in the present embodiment are
shown in FIG. 3 and FIG. 5B as tracing two separate lines 46, the
energies of these two elements can be combined on the imaging
surface, as earlier described, to form a single line of which the
position is controllable by appropriate setting of the energies
emitted by each of the additional elements 42. This is shown in
FIG. 6 in which the energy profiles of the lines 44a and 44b are
designated 94a and 94b, respectively and the energy profiles of the
additional lines 46 are designated 96a and 96b. In FIG. 6, neither
of the profiles 96a and 96b (shown in dotted lines) has sufficient
energy to render the coating particles tacky but at the centerline
between the two arrays the cumulative energy, shown as a solid dark
line 96, is sufficient to soften the particles coating and to
create a trace line filling the gap between the trace lines 44a and
44b of the two main arrays.
While in FIG. 6 the energy profiles of the two additional elements
are matched, it is possible by varying the relative intensity of
the two beams emitted by the additional laser sources to position
the centerline of the combined energy at a different distance from
the traces of the main arrays.
FIG. 7A shows how the ability to create dots that do not fall on
the centerlines of the energy profiles of the laser elements can be
used to advantage to achieve anti-aliasing. FIG. 7A shows the
energy profiles of four adjacent elements of the main array. The
first two profiles a and b are set at a desired level, say 8 (out
of sixteen), corresponding to mid-grey. The energy profiles c and
d, on the other hand are set to say 12 and 4, respectively. The
resulting dot pattern produced on the imaging surface is shown in
FIG. 7B. This can be seen to comprise two regular sized dots A and
B aligned with the line of symmetry of the profiles a and b in FIG.
7A, a larger sized dot C aligned with the centerline of energy
profile c, and a smaller dot D that lies somewhere between the
centerlines of the profiles c and d.
The result of repeating such a dot pattern diagonally is shown in
FIG. 8A. When this image is compared with FIG. 8B, where no
anti-aliasing steps have been taken, it will be seen that the small
dots in between regular raster line yield oblique edges that have
reduced jaggedness and produce an image that is comparable with one
achievable by a printing system having a greater image
resolution.
The interaction of energies from nearby laser elements can also be
used to compensate for missing or inoperative elements in that the
elements producing the two adjacent raster lines can be used to
combined in the same manner as previously explained to fill in a
gap between them.
For the arrays 130a and 130b in FIG. 3 to function correctly as
described above, their relative position in the Y-direction is very
important. In order to simplify the construction of the lens system
serving to focus the emitted laser beams on the imaging surface it
is advantageous to adopt a configuration shown in FIG. 4 which
enables the two rows of lenses corresponding to a pair of chip rows
to be self-aligning.
FIG. 4 shows arrays of seven adjacent chips 130 each shown lined up
with a respective lens 18. Additional laser elements 42, on each
side of the main array of each chip, are also schematically
illustrated in the figure. Each lens 18 is constructed as a GRIN
(Gradient-Index) rod, this being a known type of lens that is
shaped as a cylinder having a radially graduated refractive index.
In the case of the geometry shown in FIG. 4, the respective centers
of corresponding elements of any three bi-directionally adjacent
chip arrays 130 lie nominally on the apices of an equilateral
triangle, three such triangles designated 50 being shown in the
drawing. It will be noted that all the triangles 50 are congruent.
As a result, if the diameter of the GRIN rods is now selected to be
equal to 2NA.sub.r, which is the length of the sides of the
equilateral triangles 50, or the distance between corresponding
laser emitting elements of adjacent VCSEL chips 30 in the same row,
then when stacked in their most compact configurations, after
aligning the lens array to the Y-direction over the chips, the
lenses 18 will automatically align correctly with their respective
chip. For such construction, the relationship between the rod lens
diameter D, the image resolution I.sub.r and the size of the matrix
of laser elements is: D=2I.sub.rMN where I.sub.r is the spacing in
the Y-direction between adjacent lines traceable in the X-direction
and M is the number of rows and N the number of columns in the main
MN array, assuming absolute magnification value of |1|.
Though the lens 18 has been schematically illustrated in FIG. 1
(side view) and FIG. 4 (cross section view) as being an individual
GRIN rod, in alternative embodiments the laser beams of each chip
can be transmitted by a series of lenses. In the simplified
embodiment shown in FIG. 9, the single GRIN rod 18 is replaced by
two mutually inclined GRIN rods 18a and 18b and the light from one
is directed to the other by a reflecting member which in the
example of FIG. 9 is embodied by a prism 87 of high refractive
index glass, so that the light follows a folded path. It is noted
that other reflecting members such as mirrors and the like may be
utilized. Such a configuration enables coating stations in a colour
printing system to be arranged closer to one another in a more
compact configuration. Such a folded light path can adopt different
configurations while fulfilling all the requirements of
magnification and light transmission. To enable the light path to
be split in this manner, the length of the GRIN rods is preferably
selected such that light beams are individually collimated on
leaving the rods 18a and entering the rods 18b as shown by the
light rays drawn in FIG. 9.
The radiation guided by GRIN rod 18a, the proximal end of which is
arranged at a distance WD.sub.o from the chip, may be captured by
the corresponding GRIN rod 18b which can collect the collimated
light emerging from rod 18a on the same light path and focus it at
a distance WD.sub.i from the distal end of the second GRIN rod 18b.
When the two GRIN rods are made of the same material and the same
radial gradient profile and WD.sub.o=WD.sub.i a magnification of
M.sub.o=+1 or -1 can be obtained.
Notably, with straight or folded path light paths, the
magnification should be considered substantially equal to its
nominal value if within .+-.0.5% or even 1% or 2%.
Laser elements that are away from the longitudinal axis of the GRIN
rod 18a will leave the distal end of the GRIN lens collimated but
at an angle to the axis. In certain cases, it is necessary for the
distance between the two rods 18a and 18b to be large, causing the
off axis collimated beams exiting the first rod segment to miss
partially or entirely the second segment. It is possible to take
advantage of Snell's law and cause the beam exiting the first rod
to travel through a glass with a high refractive index, thus
causing the angle the collimated beam makes with the optical axis
to decrease and enabling a larger separation between the rods
before the collimated beams leaving the first rod miss the entrance
to the second rod.
In the description and claims of the present disclosure, each of
the verbs, "comprise" "include" and "have", and conjugates thereof,
are used to indicate that the object or objects of the verb are not
necessarily a complete listing of members, components, elements,
steps or parts of the subject or subjects of the verb.
As used herein, the singular form "a", "an" and "the" include
plural references and mean "at least one" or "one or more" unless
the context clearly dictates otherwise.
Positional or motional terms such as "upper", "lower", "right",
"left", "bottom", "below", "lowered", "low", "top", "above",
"elevated", "high", "vertical", "horizontal", "backward",
"forward", "upstream" and "downstream", as well as grammatical
variations thereof, may be used herein for exemplary purposes only,
to illustrate the relative positioning, placement or displacement
of certain components, to indicate a first and a second component
in present illustrations or to do both. Such terms do not
necessarily indicate that, for example, a "bottom" component is
below a "top" component, as such directions, components or both may
be flipped, rotated, moved in space, placed in a diagonal
orientation or position, placed horizontally or vertically, or
similarly modified.
Unless otherwise stated, the use of the expression "and/or" between
the last two members of a list of options for selection indicates
that a selection of one or more of the listed options is
appropriate and may be made.
The imaging device is described herein mainly by reference to its
application in digital printing systems however its use is not
limited to this application, and different aspects of the invention
may be implemented to project light beams onto any surface with
relative motion between the surface and the chips.
In the disclosure, unless otherwise stated, adjectives such as
"substantially" and "about" that modify a condition or relationship
characteristic of a feature or features of an embodiment of the
present technology, are to be understood to mean that the condition
or characteristic is defined to within tolerances that are
acceptable for operation of the embodiment for an application for
which it is intended. For instance, each two adjacent elements of
the group of elements under consideration (such as by way of
example of a chip row, of a chip column, or of adjacent chip
arrays, when applicable) are considered "substantially uniformly
spaced" if the deviation of each pair of adjacent elements from a
desired nominal distance does not exceed 10% of this predetermined
spacing. Pairs of adjacent elements deviating from the nominal
distance by less than 5%, 4%, 3%, 2% or 1% are further considered
"substantially uniformly spaced" or "having a substantially uniform
spacing". By way of example, assuming a desired A.sub.r=20
micrometers, and the desired nominal spacing in the Y-direction
between corresponding main array laser emitting elements in two
adjacent chips equals A.sub.rN, spacing deviations resulting from
manufacturing tolerance of no more than 2 .mu.m, are considered to
fall within the nominal spacing. Clearly, smaller or no deviations
are desired.
While this disclosure has been described in terms of certain
embodiments and generally associated methods, alterations and
permutations of the embodiments and methods will be apparent to
those skilled in the art. The present disclosure is to be
understood as not limited by the specific embodiments described
herein.
* * * * *